(i) Field of the Invention
This invention relates to a method for reducing the proportion of trimethylamine by a disproportionating reaction, said trimethylamine being by-produced in a small amount upon preparation of dimethylamine and monomethylamine through a reaction between methanol and ammonia in the presence of a shape-selective catalyst. More specifically, this invention pertains to a catalyst useful in allowing the disproportionating reaction to proceed efficiently and also in suppressing by-production of trace components which are formed in the disproportionating reaction.
(ii) Description of the Related Art
As catalysts for the disproportionation of trimethylamine, those having solid acidity are known to show catalytic ability for many years. As those exhibiting solid acidity, alumina, silica-alumina, silica-magnesia, alumina-titania, silica-titania, silica-zirconia, alumina-zirconia, solid phosphoric acid, large pore zeolites and the like are known to be effective for the disproportionating reaction. Among these solid acid catalysts, proposed and widely employed are those composed primarily of silica and/or alumina, specifically, silica-alumina catalysts (Japanese Patent Laid-Open No. 169445/1982, Japanese Patent Publication No. 47172/1987, U.S. Pat. No. 4,485,261) and rhenium-ion-exchanged zeolite Y (REY zeolite), i.e., "SK-500" (trade name; product of Union Carbide Corporation, U.S.A.; U.S. Pat. No. 4,398,041).
The above-described conventional catalysts proposed to date do not have sufficient catalytic activities. A reaction temperature of 375.degree. C. or higher is needed to achieve sufficient conversion of trimethylamine especially with an amorphous silica-alumina catalyst. This disproportionating reaction is an equilibrium reaction. Although a somewhat higher temperature is advantageous for the conversion at equilibrium, it may be sufficient in balance if the conversion at equilibrium available at about 300.degree. C. is obtained. There is a demand for a catalyst which exhibits sufficient activities in a temperature range of about 280.degree.-380.degree. C., because this disproportionating reaction is an endothermic reaction and therefore, heat of reaction should be supplied from the outside to the catalyst layer. The lower the reaction temperature, the more advantageous in both apparatus and energy for supplying heat of reaction. From such a viewpoint, the above-described REY zeolite has the advantage that it shows activities at a lower temperature than an amorphous silica-alumina catalyst. According to the results of a test conducted by the present inventors, however, REY zeolite has been found to involve the problem that reaction by-products--for example, acetonitrile, acetone, propionitrile and propyl-amine--are formed in trace amounts. These trace by-products accumulate in the course of separation and purification of the reaction products, and cause the problem that the efficiency of the separation is lowered.